Isn't decorating a "fake cake", i.e. a dummy cake, and then making a sheet cake just as much work and almost as many ingredients as doing a real cake from the start? It's not a great way to save money and it can be tacky, IMO. I would charge as much to do a dummy and then a sheet cake as I would to do a real tiered cake.

For me, it usually actually works out to be MORE expensive for a faux cake plus kitchen cakes than if they just go with the real cake. I charge 80% of my normal price for dummy cakes, plus the kitchen cakes. When I break everything down for brides, they always decide against the faux cake idea. On the FAQ page of my website here's how I address the myth of fake cakes:

Isn't it cheaper to get a fake display cake and serve sheet cakes at my wedding?

No. It takes just as much time and icing to decorate styrofoam as it does cake. Then you'll have to pay for the kitchen cakes, which cost almost the same as wedding cake. You will actually end up paying more for a display cake and kitchen cakes than you would if you orderd the appropriate servings of wedding cake. Take this example: